1. Field of the Invention
The present invention relates to an exhaust gas control catalyst.
2. Description of Related Art
An exhaust gas control catalyst of an automobile causes hydrocarbon (HC) and carbon oxide (CO) to be oxidized into water and carbon dioxide and causes nitrogen oxide (NOx) to be reduced into nitrogen, in which hydrocarbon (HC), carbon oxide (CO), and nitrogen oxide (NOx) are contained, in exhaust gas exhausted from an engine. As the exhaust gas control catalyst, a noble metal-supported catalyst in which a catalyst layer containing particles of catalytic noble metals such as palladium (Pd), rhodium (Rh), and platinum (Pt) is supported on a heat-resistant metal support is commonly used.
The catalyst layer of the noble metal-supported catalyst contains an OSC material having oxygen storage capacity (hereinafter, also referred as “OSC”) as a co-catalyst in addition to the above-described catalytic noble metal particles. The OSC material stores and releases oxygen to promote an exhaust gas control reaction by the catalytic noble metals. As the OSC material, ceria (CeO2)-zirconia (ZrO2) solid solution is widely used, but various OSC materials have been being developed for further performance improvement.
For example, Japanese Patent No. 4670603 describes catalyst particles which have a function of storing and releasing oxygen, the particles including: an iron compound in which an element other than Fe having two valences is solid-soluted in an oxide of Fe having three valences; and a core portion formed from an oxide of Fe having three valences as a main component, in which the iron compound constitutes a layer formed on the outer peripheral surface of the core portion.
Japanese Patent No. 4666006 describes an exhaust gas control catalyst in which a catalyst layer is formed on a support, the catalyst layer including: Ce-containing oxide particles having oxygen storage capacity; and a catalytic metal formed of at least one noble metal selected from the group consisting of Pt, Pd, and Rh. In the catalyst layer, a large amount of iron oxide particles are dispersed and contained, at least a part of the iron oxide particles are fine iron oxide particles having a particle size of 300 nm or less, the fine iron oxide particles are in contact with the Ce-containing oxide particles, an ratio of the area of the fine iron oxide particles to the total area of the iron oxide particles is 30% or higher when observed with an electron microscope.
Japanese Patent No. 4525909 describes a catalyst for a water gas shift reaction which is formed from a composite oxide. This catalyst contains calcium and aluminum as major components and contains iron in an amount of 0.001 mol % to 10 mol % with respect to aluminum. In the catalyst, an oxygen storage capacity at 500° C. is 20 μmol/g to 200 μmol/g, and a BET specific surface area value is 20 m2/g or higher.
As described above, various materials containing iron oxide have been developed as an OSC material used for an exhaust gas control catalyst. However, when an exhaust gas control catalyst which contains a material containing iron oxide as an OSC material is used over a long period of time, the catalytic activity of a catalytic noble metal may decrease, and/or the oxygen storage capacity of an OSC material may decrease.